1. Field of the Invention
The pulsed doppler device is commonly used in the radar frequency front end of moving target indicating radar systems. Such a device obtains range resolution via pulse transmission in a conventional manner, and a doppler principle (target phase information) is used to separate moving targets from the background.
It is desirable to operate the radar with the highest possible duty ratio ("on" to "off" ratio) in order to maximally utilize the transmitter average power capability. This practice almost universally produces the lowest transmitter cost. For short range radars (a few miles of range) at UHF, the high duty ratio also commonly allows solid state construction, affording desirable improvements particularly in reliability and size. For conventional pulsed doppler radar, however, a high duty ratio either translates to ambiguous range responses which require special processing for their resolution, or such poor range resolution results that the apparatus loses usefulness.
2. Description of the Prior Art
Conventional radar systems can generally be divided into three types, as follows: pulsed doppler, continuous wave (CW), and interrupted continuous wave (ICW).
In a conventional pulsed doppler radar, a series of pulses are transmitted, each pulse having a predetermined amplitude, duration, and frequency. In order to maintain adequate average power for acceptable target size resolution, pulsed doppler radar employs a high pulse repetition frequency (PRF). High PRF, however, undesirably limits the unambiguous range of the radar which is a function of the time separation between two adjacent pulses.
A continuous wave (CW) radar overcomes the problem of range ambiguity to some extent by transmitting a continuous wave. The generated wave is transmitted through a first antenna and the reflected wave from a target is received through a second antenna. The generated wave is typically modulated with a distinct code so that the time differential between the transmitted wave and the received wave can be measured.
A short-coming of the continuous wave radar is that, as power or distance to the target increase, the transmitted signal leaks directly from the transmitting antenna to the receiving antenna in sufficient magnitude to interfere with the reception of the signal returning from the target.
The interrupted continuous wave (ICW) radar, sometimes referred to as the intermittent continuous wave, is designed to overcome the leakage problem of the continuous wave radar. In ICW, the coded bits are transmitted individually with sufficient time between the pulses to allow the receiving antenna to receive the reflected pulses. The leakage between the transmitting and the receiving antennae is thus overcome. The difficulty with ICW radar is that, for an adequate unambiguous range, the separation between individual pulses is rather long, resulting in a low average power of the transmitted signal. The "on" time of each pulse typically represents only about 3% of the duty cycle.